Enable an AVX2 build of glibc core libraries.
Note - the dynamic linker itself should be omitted from the avx2 directory, but the core glibc libs (math, etc) should have an AVX2 variant by
default to squeeze some extra juice out of the system.
At some point we should also do the same to Mesa, but this will require us to first move to glvnd and ypkg-ify our mesa, moving legacy comar
out to kernel-glue.